Unionized teaching assistants at several Southern California campuses walked off the job Thursday as part of a one-day strike.

The strikes took place at University of California campuses in Los Angeles, Irvine, Santa Barbara and Riverside. Other campuses went on strike the day before. UC employs about 13,000 unionized teaching assistants, (TAs), tutors, and undergraduate readers; but according to students and campus officials, only a small percentage failed to show up for work.

UCLA doctoral student Nicole Robinson took part in a campus rally and march. She’s upset her union and management haven’t been able to agree on an increase to the TA’s roughly $17,000 yearly stipend.

“And we’re also protesting against huge class sizes,” she said, adding that it has created more work for graduate students at a time when they need to focus on their own research.

Officials at UCLA and UC Irvine said disruption to classes was minimal.

At UC Irvine, several hundred people attended a strike rally on Thursday. Graduate students there said a handful of classes were cancelled following the walkout. Cathy Lawhon is UC Irvine’s spokeswoman.

“The disappointment on the university’s part is that these workers would walk off the job and inconvenience and jeopardize the education of our students here while the talks are still going on,” she said.

The strike is also a reaction to what teaching assistants called university intimidation of graduate students involved in union organizing at several campuses. UC Irvine graduate students said a November 2013 protest on campus was videotaped by campus police.

“The incident that they were talking about was something where they were blocking a public street and so, at that point our police did videotape some of them who refused to leave the public street,” Lawhon said.

The next contract talks between UC and the United Auto Workers, the union representing the TAs, are scheduled for April 15 and 16.